Vomiting (also called throwing up or emesis) is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting may result from many causes, ranging from gastritis or poisoning to brain tumors, or elevated intracranial pressure (ICP). The feeling that one is about to vomit is called nausea. It usually precedes, but does not always lead to vomiting. Antiemetics are sometimes necessary to suppress nausea and vomiting, and, in severe cases where dehydration develops, intravenous fluid may need to be administered to replace fluid volume.

The medical branch investigating vomiting, emetics and antiemetics is called emetology.

Vomiting is different from regurgitation, although the two terms are often used interchangeably. Regurgitation is the return of undigested food (that has not yet reached the stomach) back up the esophagus to the mouth. The causes of vomiting and regurgitation are generally different.

Retroperistalsis, starting from the middle of the small intestine, sweeping up the contents of the digestive tract into the stomach, through the relaxed pyloric sphincter.

A lowering of intrathoracic pressure (by inspiration against a closed glottis), coupled with an increase in abdominal pressure as the abdominal muscles contract, propels stomach contents into the esophagus without involvement of retroperistalsis. The lower esophageal sphincter relaxes. This is part of the motor output, and it is also important to note that the stomach itself does not contract in the process of vomiting.

Gastric secretions and likewise vomit are highly acidic. Recent food intake will be reflected in the gastric vomit. Irrespective of the content, vomit tends to be malodorous.

The content of the vomitus (vomit) may be of medical interest. Fresh blood in the vomit is termed hematemesis ("blood vomiting"). Altered blood bears resemblance to coffee grounds (as the iron in the blood is oxidized), and, when this matter is identified, the term "coffee ground vomiting" is used. Bile can enter the vomit during subsequent heaves due to duodenal contraction if the vomiting is severe. Fecal vomiting is often a consequence of intestinal obstruction or a gastro-colic fistula, and is treated as a warning sign of this potentially serious problem ("signum mali ominis"); such vomiting is sometimes called "miserere."

If the vomiting reflex continues for an extended period of time with no appreciable vomitus, the condition is known as non-productive emesis or dry heaves, which can be painful and debilitating.

Vomiting can be very dangerous if the gastric content gets into the respiratory tract. Under normal circumstances the gag reflex and coughing will prevent this from occurring, however these protective reflexes are compromised in persons under narcotic influences such as alcohol or anesthesia. The individual may choke and asphyxiate or suffer an aspiration pneumonia.

Prolonged and excessive vomiting will deplete the body of water (dehydration) and may alter the electrolyte status. The loss of acids leads to metabolic alkalosis (increased blood pH), and the electrolyte imbalance shows hypokalemia (potassium depletion) and hypochloremia (chlorine depletion). The hypokalemia is an indirect result of the kidney compensating for the loss of acid. With the loss of intake of food the individual will become cachectic.

Repeated or profuse vomiting may cause erosions to the esophagus or small tears in the esophageal mucosa (Mallory-Weiss tear). This may become apparent if fresh blood is admixed with vomit after several episodes.

To remove a poison in case such has been ingested (some poisons should not be vomited as they may be more toxic when inhaled or aspirated; it is, in general, considered better to ask for help before inducing vomiting)

Some people who are engaged in binge drinking will induce vomiting in order to make room in their stomachs for further alcohol consumption. In the United Kingdom, this practice is known as tactical chundering, or hitting the reset button and as boot and rally or pulling the trigger in the United States.

An emetic, such as Syrup of Ipecac, is a substance that induces vomiting when administered orally or by injection. An emetic is used medically where a substance has been ingested and must be expelled from the body immediately. Inducing vomiting can remove the substance before it is absorbed into the body. Ipecac abuse can lead to detrimental health effects.

It is quite common that, when one person vomits, others nearby will become nauseated, particularly when smelling the vomit of others, often to the point of vomiting themselves. It is believed that this is an evolved trait among primates. Many primates in the wild will tend to browse for food in small groups. Should one member of the party react adversely to some ingested food, it may be advantageous (in a survival sense) for other members of the party also to vomit. This tendency in human populations has been observed at drinking parties, where excessive consumption of alcoholic beverages may result in a number of party members vomiting nearly simultaneously, this being triggered by the initial vomiting of a single member of the party.

Intense vomiting in ayahuascaceremonies is a common phenomenon. However, people who experience "la purga" after drinking ayahuasca, in general, regard the practice as both a physical and spiritual cleanse and often come to welcome it. [2] It has been suggested that the consistent emetic effects of ayahuasca — in addition to its many other therapeutic properties — was of medicinal benefit to indigenous peoples of the Amazon, in helping to clear parasites from the gastrointestinal system. [3]

There have also been documented cases of a single ill and vomiting individual inadvertently causing others to vomit, when they are especially fearful of also becoming ill, through a form of mass hysteria.

Most people try to contain their vomit by vomiting into a sink, toilet, or trash can, as both the act and the vomit itself are widely considered embarrassing; vomit is also difficult to clean. On airplanes and boats, special bags are supplied for sick passengers to vomit into. A special disposable bag containing absorbent material that solidifies the vomit quickly is available, also, making it convenient and safe to keep (leakproof, puncture-resistant, odorless) until there is an opportunity to dispose of it conveniently.

According to an online study of 30 traditionally bad sounds, the sound of vomiting is the worst sound in the world.[4] Professor Cox of the University of Salford's Acoustic Research Centre said that "We are pre-programmed to be repulsed by horrible things such as vomiting, as it is fundamental to staying alive to avoid nasty stuff". It is thought that the thought of disgust is triggered by the sound of vomiting to protect food from those possibly diseased nearby[5].

Vomit phobia, or emetophobia, as it is also known, is the sixth-most-common phobia in the world, according to the International Emetophobia Society. In addition to the actual phobia, there are many other disorders and phobias that sufferers may be afflicted with, such as irritable bowel syndrome and agoraphobia. People with emetophobia tend to avoid eating in public, socialising, and going to parties. They may hardly eat at all, and for this reason may be diagnosed as anorexic. Emetophobes will go to extraordinary lengths to avoid vomiting or seeing someone vomit. Some emetophobics have the distinct ability to actually prevent themselves from vomiting, called "vomit continence." They are able to fight the feeling of nausea before the feeling eventually subsides.